[SciPy-Dev] splev
Joshua Holbrook
josh.holbrook@gmail....
Thu Jul 8 01:02:47 CDT 2010
On Wed, Jul 7, 2010 at 8:19 PM, Charles R Harris
<charlesr.harris@gmail.com> wrote:
>
>
> On Wed, Jul 7, 2010 at 10:06 PM, Anne Archibald <aarchiba@physics.mcgill.ca>
> wrote:
>>
>> On 7 July 2010 23:45, Charles R Harris <charlesr.harris@gmail.com> wrote:
>> > Hi All,
>> >
>> > I opened ticket #1223 because the values returned by splev are not zero
>> > for
>> > arguments outside of the interval of definition. Because splev evaluates
>> > b-splines, which all have compact support, I think zero is the correct
>> > value
>> > for such arguments. However, I also find that some tests assume that
>> > splev
>> > extrapolates the interpolating polynomials defined in the first and last
>> > spans, which is what splev currently does. Consequently I thought it
>> > worth
>> > bringing the topic up on the list for discussion before making the
>> > commit
>> > that changes the behavior.
>>
>> Please do not make this change. The extrapolation you get now is ugly,
>> as polynomial extrapolation always is, but a discontinuity is
>> considerably uglier. Remember that the arguments and range are both
>> floating-point, so that roundoff error can easily move an argument
>> outside the range; if extrapolation is used this is unimportant, but
>> if the spline flatly drops to zero this will produce wildly wrong
>> answers.
>>
>
> Well, for my purposes extrapolation really screws things up. Note that the
> b-splines at the ends are in fact c_{-1}, i.e., discontinuous, and the next
> b-spline in has a discontinuous 1'st derivative, so on and so forth. The
> discontinuities are mathematically correct, b-splines aren't polynomials.
> Maybe we can add a keyword?
>
> Chuck
>
>
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>
I personally agree with Anne. While discontinuities may be technically
correct, I think the current behavior is more forgiving. If this is
noted in the docs, I say things are g2g.
Chuck: Is checking the interval yourself prohibitive?
--Josh
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